In the processing of a semiconductor wafer to form integrated circuits, a number of traces are normally formed over an underlying layer. The traces are normally used to interconnect transistors and other devices in the integrated circuits. Such traces may have widths under 0.2 micrometers (microns), pitches (center to center spacing) under 0.4 microns and aspect ratios exceeding 4:1.
Depending on the stage of the processing, it may be necessary to measure properties of various portions of a wafer, such as the properties of the traces and/or the properties of the underlying layer. However, the presence of traces can interfere with conventional measurements that examine open areas (areas not covered by traces).
A structure having a number of lines supported by a layer in contact with the lines (also called xe2x80x9cmulti-layered structurexe2x80x9d) is evaluated in accordance with the invention by illuminating a region (also called xe2x80x9cilluminated regionxe2x80x9d) containing several lines, using a beam of electromagnetic radiation, and generating an electrical signal (e.g., by use of a photosensitive element) that indicates an attribute (e.g., intensity or optical phase) of a portion (also called xe2x80x9creflected portionxe2x80x9d) of the beam reflected from the region. As more than one line (and therefore more than one portion of the layer in contact with the lines is being illuminated, the reflected portion and the electrical signal generated therefrom do not resolve individual features in the illuminated region, and instead indicate an average measure of a property of such features. In contrast, most prior art methods measure a property of an individual feature in such a multi-layered structure. The just-described lines can be either conductive (in which case they are also referred to as xe2x80x9ctracesxe2x80x9d) or non-conductive, depending on the embodiment.
In one embodiment, the acts of xe2x80x9cilluminatingxe2x80x9d and xe2x80x9cgeneratingxe2x80x9d are repeated in another region (of the same structure or of a different structure) also having multiple traces. The electrical signals being generated from light reflected by different regions can be automatically compared to one another to identify variation of an average property (e.g., average thickness of the layer in contact with the traces, or average resistance per unit length of the traces) between the regions. Instead of (or in addition to) the just-described comparison, the values of such a signal can be plotted in a graph to indicate a profile of a surface in the region. A value being plotted can be an absolute value of the reflected portion alone, or can be a value relative to another portion that is reflected by another surface in the same region (which indicates the average distance therebetween), or by the same surface in another region (which indicates an average profile of the surface).
Such measurements can identify variations in properties in a semiconductor wafer of the type used in fabrication of integrated circuit dice, or between multiple such wafers (e.g., values measured from a reference wafer and a production wafer or between two successive production wafers can be compared). Identification of a change in a property between two or more wafers is useful e.g., when performing such measurements during wafer fabrication, so that process parameters used to fabricate a next wafer (e.g.; creating the above-described layer or the traces) can be changed as necessary (in a feedback loop), to generate wafers having material properties within acceptable limits. Note, however, that structures other than semiconductor wafers (e.g., photomasks that include a glass substrate and are used to form the wafers, or an active matrix liquid crystal display) can also be evaluated as described herein.
In a first example, there is a transmissive medium (such as air) located between a source of the beam (also called probe beam) and the illuminated region. In one implementation, another beam (also called xe2x80x9cheating beamxe2x80x9d) is used in addition to the probe beam, to modulate the temperature of the traces (e.g., at a predetermined frequency). Reflectance of the lines changes with the change in temperature. The reflected portion (which depends on reflectance), and hence the generated signal also oscillates (e.g., as the predetermined frequency). Such an oscillating signal is measured by e.g., a lock-in amplifier, and the measurement is repeated in another region. If all lines in the illuminated region are conductive (also referred to as xe2x80x9ctracesxe2x80x9d), comparison of measurements from different regions (e.g., which may be in the same location in different die of a wafer, or which may be in the same die in different wafers) indicates a change in the average resistance per unit length (and therefore the corresponding change in cross-sectional area) between traces in the respective regions (if conductivity is constant).
A series of measurements from regions adjacent to one another (or even overlapping one another) in the longitudinal direction of the traces, when plotted in a graph along the y axis with the x axis indicating distance along the longitudinal direction yields a profile of the traces (which may be used to detect, e.g. global nonuniformity such as a dimple or a dome). Depending on the specific variant, the probe beam and the heating beam can each be coincident with or offset from the other.
In another implementation, multiple traces in a region of a structure of the first example are each substantially parallel to and adjacent to the other, and the beam has wavelength greater than (or equal) to a pitch between two adjacent traces. In one such embodiment, the probe beam is polarized (e.g., by a polarizing optical element interposed between a source of the beam and the structure), although a nonpolarized probe beam can be used in other embodiments. A polarized probe beam can be used in several ways, including, e.g., orienting the probe beam so that the electrical field vector for the electromagnetic radiation is at a predetermined angle relative to the traces.
When the probe beam is polarized perpendicular to the traces, the traces do not reflect the probe beam. Instead, the probe beam passes between the traces and is reflected from underneath the traces, e.g. by charge carriers of a semiconductor layer, or by a surface of an oxide layer, or both. Such light which is reflected from underneath the traces can be used to identify variation in a property of features underneath the traces (averaged over the features that are illuminated). The portion reflected by charge carriers is relatively small (e.g., {fraction (1/10)}4 or less) as compared to the portion reflected by an underlying surface, and therefore has a negligible effect on an overall measurement of a steady signal (also called xe2x80x9cDCxe2x80x9d component). If necessary, the portion reflected by charge carriers can be measured by modulating the number of charge carriers and using a lock-in amplifier to measure the portion of a reflected light that is modulated (also called xe2x80x9cACxe2x80x9d component) as described elsewhere herein. The charge carriers can be created by a beam having an oscillating intensity (or oscillating phase). In this variant, the reflected portion has an intensity (or phase) that is modulated in phase with modulation of the charge carriers (and can be measured by use of a lock-in amplifier).
When the probe beam is polarized parallel to the longitudinal direction of the traces, the above-described reflected portion (that is used to generate the electrical signal) is reflected by the traces. The reflected portion can be used to identify variation in a property that is averaged over the traces. A probe beam polarized parallel to the traces can be used with a heating beam that is also polarized parallel to the traces, and in such a case effectively on the traces interact with the heating beam, and are heated more, as compared to heating by an unpolarized heating beam. Alternatively, the just-described probe beam (also called xe2x80x9cparallel polarized beamxe2x80x9d) can be used with another probe beam that is polarized perpendicular to the traces (also called xe2x80x9cperpendicular polarized beamxe2x80x9d). The two polarized beams can be generated from the same beam, e.g., by a polarizer or a polarizing optical element (such as a Wollaston beam splitter), or by a combination of such optical elements (e.g. Wollaston beam splitter followed by a polarizer). A polarizer here refers to any optical element or set of optical elements whose output is a beam with a single direction of polarization.
In one embodiment, a portion of the parallel polarized beam reflected by the traces, and a portion of the perpendicular polarized beam reflected from underneath the traces interfere, and the interference pattern is used to generate an electrical signal. As noted above, the electrical signal indicates a profile of the underneath surface when the beams are offset. When the parallel polarized beam and the perpendicular polarized AD beam are coincident, the electrical signal indicates a distance between the underneath surface and a surface of the traces exposed to the transmissive medium (also called xe2x80x9cexposed surfacexe2x80x9d). Note that the exposed surface of the structure can be formed by a surface of the traces and a surface of the layer that interdigitates between the traces (the layer surface and the trace surface can be substantially co-planarxe2x80x94within the same plane or in planes that are separated from each other by less than 10% of the width of the traces) and such surfaces can be formed, e.g., by chemical mechanical polishing.
The two probe beams that are polarized mutually perpendicular to each other can each be oriented at 45xc2x0 relative to the traces, so that at least a portion of each beam is reflected from the exposed surface of the structure. In such a case, the electrical signal obtained from the two or more reflected portions indicates a profile of the exposed surface, assuming the two beams are offset from one another, and the surface containing the traces has a constant profile. An optional polarizing beam splitter can be used to limit the measurement to the two portions that are reflected by the traces (or to the two portions that are reflected by a surface underneath the traces when profiling the underneath surface). Therefore, illuminating a region containing two or more traces allows use of the wafer as a polarizer to measure an average property of features underneath the traces that are otherwise inaccessible.
In a second example, the traces are separated from the transmissive medium by a layer (also called xe2x80x9cexposed layerxe2x80x9d) included in the structure. One method used with the second example measures a signal obtained from interference between a portion of the probe beam reflected by the traces, and another portion reflected by a layer formed over the traces. Reflection of a perpendicular polarized beam by the exposed layer overcomes a prior art problem of illuminating a region containing traces, because the traces do not adversely affect the perpendicularly polarized light (e.g., the traces reflect parallel polarized light). The just-described method does not require a heating beam. This method also has the advantage of being able to measure a property of traces buried underneath the exposed layer.
In a variant of the just-described method, both portions are reflected by the traces, and each portion is offset from the other thereby to yield a signal indicative of a profile of the surface of traces (although the traces are located underneath the exposed layer). In such a method, the to-be-reflected portions of a probe beam can be polarized mutually perpendicular to each other and oriented at 45xc2x0 relative to the traces. Instead of mutually perpendicularly polarized beams, two beams that are polarized parallel to one another and also parallel to the traces also can be used, e.g., to obtain a surface profile of the traces (that are located underneath the exposed layer).
Furthermore, instead of being offset from one another, the parallel polarized beams can be coincident, with one beam being the probe beam and the other beam being the heating beam. In such a case, the measured signal provides an indication of a property of the traces, although the traces are located underneath the exposed layer. If the two beams that are polarized parallel to one another (e.g., a probe beam and a heating beam) are both oriented perpendicular to the traces (a first set) underneath the exposed layer, a property of a second set of traces located underneath the first set can be determined. Furthermore, instead of a heating beam, a pump beam can be used to generate charge carriers in a layer located underneath the traces.
One implementation combines two of the above-described methods, by using two beams that are respectively polarized parallel and perpendicular relative to the longitudinal direction of a set of traces in the structure. In this implementation, two electrical signals for two measurements in the two polarization directions are generated contemporaneously (e.g., just before, during or just after each other). Simultaneous generation of the two electrical signals provides an advantage in speed, as compared to sequential generation of the two signals. Such electrical signals can provide measures of properties of both traces and a layer underneath the traces, so that a wafer can be accepted or rejected in a signal operation.
In another embodiment, the probe beam is nonpolarized (or has circular or elliptical polarization so that both orthogonal polarization components are simultaneously pre sen t in the single probe beam). In one implementation of this embodiment, the method includes generating a single electrical signal from the portion of light reflected when a nonpolarized (or circular or elliptical polarized) probe beam is used. In another implementation, the method includes contemporaneous generation of two electrical signals based on measurement of two components of the reflected portion: a first component that is polarized in a direction perpendicular to the traces, and a second component that is polarized in a direction parallel to the traces.